EP0805942B1

EP0805942B1 - Apparatus for the transfer of heat with the aid of air

Info

Publication number: EP0805942B1
Application number: EP96902509A
Authority: EP
Inventors: René Jean Marie VAN GERWEN; Bartel Jan Cornelis Van Der Wekken
Original assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Current assignee: Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO
Priority date: 1995-01-24
Filing date: 1996-01-24
Publication date: 2001-04-18
Anticipated expiration: 2016-01-24
Also published as: EP0805942A2; WO1996023188A2; WO1996023188A3; DE69612546T2; AU4679296A; NL9500130A; DE69612546D1

Description

The present invention relates to an apparatus for the transfer of heat with the aid of air according to the preamble of Claim 1.
Such an apparatus has been disclosed by US-A-4.665.715, corresponding to EP-A-0.192.501. This US-A-4.665.715 discloses the following process and apparatus for it: by means of a suction line outside air is drawn in by means of a compressor, which delivers the air via a line to a heat exchanger. From this exchanger the air is delivered via a line to a water extractor. From the water extractor it goes to an expansion turbine and from the expansion turbine it goes to the enclosure room. From the disclosure the air is discharged via a line to heat exchanger 23. From this heat exchanger 23 it is discharged to the outside.
An other apparatus for the transfer of heat with the aid of air is disclosed by US-A-4.769.051. According to US-A-4.769.051 air is taken in from the environment by means of an engine and supplied to a compressor via a primary heat exchanger. After the compressor, the air is supplied to a regenerative heat exchanger via a secondary heat exchanger 24. After the regenerative heat exchanger, the air passes through a water collector, through a filter and a unit before reaching a turbine. From the turbine the air passes through a load heat exchanger and afterwards through the regenerative heat exchanger to exit the cycle via the unit into the environment. The extraction of heat from the air within the closed environment, the target room, takes place by means of the load heat exchanger. The second air cycle is not in fluid connection with the first air cycle.
JP-A-57.174.692 discloses the use of balls of glass, steel or the like as heating medium in a regenerative heat exchanger.
A still further apparatus for the transfer of heat with the aid of air is disclosed by journal 'VMT VOEDINGSMIDDELTECHNOLOGIE', vol. 25, no. 21, 8 October 1992 (NL) Zeist, pp. 75-77 'Koelproces zonder CFK's: de air cycle [Cooling process without CFCs: the air cycle]' by R.J.M. van Gerwen and S.M. van der Sluis. The apparatus described therein is configured as a refrigerating plant, that is to say the first means comprise a compressor and the second means comprise an expander, the first part of the regenerator is designed to take up heat from the air and the second part of the regenerator is designed to give up heat to the air. The outlet of the second part of the regenerator is connected to the inlet of the first means, or compressor, via a heat exchanger of conventional design. As can be seen from Fig. 1 of this publication, it is also possible to place the heat exchanger downstream of the compressor. Such an apparatus has the disadvantage that, at cooling temperatures below 0°C, the regenerator freezes up, as a result of which defrosting agents have to be added. In order to prevent the regenerator from freezing up, it is necessary for the gas used, such as air, either to contain little moisture or to be used at a relatively high temperature. In the case of the apparatus described in said journal, however, the gas or air is passed through the target room or cold-storage room. This is essential for the 'Joule-Brayton' system and differentiates this system from conventional closed systems which use CFCs, ammonia and the like. This means that a change in the composition of the gas has a direct effect on the contents of the target room. Changing the temperature of the gas is not possible, since this would lead to an unacceptable change in the temperature. The apparatus configured as a heat pump has the disadvantage that, at outside temperatures below 0°C, the heat exchanger freezes up, as a result of which defrosting agents have to be added.
The aim of the present invention is to provide an improved apparatus wherein said freezing and the additional defrosting agents resulting therefrom can be dispensed with, wherein the energy efficiency of the apparatus is increased, wherein the thermal efficiency of the regenerator is increased, wherein the quality of the frozen products is better maintained and which can be implemented more simply and more cost-effectively.
This aim is achieved in an apparatus described hereinabove having the characterizing features as described in Claim 1.
By omitting the heat exchanger between the outlet of the regenerator and the inlet of the first means for changing the pressure of the air, that is to say by drawing in the air into said first means directly from the surroundings and blowing off the air from the outlet of the regenerator directly into the surroundings, the temperature difference across this heat exchanger which is no longer present is prevented, as a result of which the apparatus can operate with a higher energy efficiency. In the apparatus configured as a heat pump, the heat exchanger which is no longer present is prevented from freezing up, as a result of which the heat pump can continue to operate even at outside temperatures below 0°C.
According to the invention, transfer of both sensible and latent heat, the latter in the form of condensation, freezing, sublimation, desublimation, defrosting or evaporation, takes place in the regenerator, As a result, the energy exchange in said regenerator is extremely efficient. Due to the fact that the stream of air which is blown into the cold-storage room is saturated with water vapour, frozen food products do not dry out and the quality is maintained.
For the benefit of the sufficiently large heat-exchange surface, it has been found to be advantageous according to the invention if the equivalent diameter D of the particles of the particulate material is between 1 and 25 mm. The equivalent diameter D is here defined as D = 3 6V π in which V is the volume of a particle.
The regenerative heat exchanger of the apparatus according to the invention functions very well by using as a basis the design requirement that the physical properties of the particulate material, the equivalent diameter of the particulate material, and the dimensions of the heat exchanger have to satisfy the following relationship: B = A D λ2m air CpL2 < 0.1 wherein

A =: total heat-exchanging surface of the particles in one compartment, in m²
D =: average equivalent diameter of the particles in m
λ =: heat transfer coefficient of the particles in W/mK
m_air =: mass flow of air in kg/s
C_p =: specific heat capacity of air in J/KgK
L =: length of bed in m.

The change in state in the regenerator now preferably takes place in the regenerative heat exchanger, the particulate material satisfying the requirements as described in Claims 4-8. Tests have shown that with such materials no blockage of the bed takes place, while on the other hand optimum regenerative properties are achieved. More particularly, it is particularly advantageous according to the invention if the ratio of the volume of particulate material to the volume occupied by the bed is between 0.2 and 0.8. With such a ratio, it is possible to realize a good flow through the bed and, at the same time, a good exchange of sensible and latent heat and vapour.
Very good results can be achieved in particular using spherical particles.
On the basis of the above formula, many types of material can be used as particulate material. It is, however, particularly advantageous according to the invention if the particulate material comprises one or more of the following materials: ceramic material, glass, and/or hygroscopic material. Here, the particles can per se be composed of one or more of these materials, but it is also conceivable to construct the bed from a mixture of various particles, for example a mixture of glass particles and ceramic particles.
According to an advantageous embodiment, wherein the regenerative heat exchanger comprises changeover means in order to be able to incorporate one compartment alternately in one or the other duct system, the regenerative heat exchanger comprises two compartments, which can simultaneously be incorporated in one or the other duct system, respectively. Such a regenerative heat exchanger can be operated essentially continuously, the particulate material taking up heat from the passing flow of fluid in one compartment, while in the other compartment heat is given up to the flow of fluid passing through there by the particulate material. As soon as the particulate material has taken up sufficient sensible and latent heat in one compartment and the particulate material has surrendered its sensible and latent heat in the other compartment, the two flows of fluid and the two compartments can be switched over with respect to one another. This alternating process can then be continually repeated.
According to an embodiment which is favourable in terms of energy, the apparatus comprises further compression means, the outlet of which is connected to the inlet of the first compression means, optionally via a heat exchanger exchanging heat with the surroundings. Advantageously, the compression means and the expansion means in this case each comprise a rotor, it being possible to drive the two rotors by means of a common shaft. Such a combined compression-expansion unit (a so-called turbo charger) generally operates optimally at rotational speeds at which it is hardly possible to supply the work required directly to the common shaft. This extra work required can be supplied by means of the further compression means.
The apparatus according to the invention can be used very advantageously in a refrigerating plant or heat pump for cooling or heating, respectively, a target room, the target room preferably being in open communication with the refrigerating plant or heat pump, respectively.
The invention will be explained in more detail below with reference to a few exemplary embodiments shown in the drawing, in which:
Fig. 1a shows a diagrammatic view of a refrigerating plant according to the invention;
Fig. 1b shows a diagrammatic view of a so-called heat pump according to the invention;
Fig. 2a shows a refrigerating plant according to a further embodiment of the invention;
Fig. 2b shows a heat pump according to a further embodiment of the invention;
Fig. 3 shows diagrammatically a regenerative heat exchanger according to the invention of the so-called static type, and
Fig. 4 shows diagrammatically a regenerative heat exchanger according to the invention of the so-called rotary type.
In Fig. 1a, a refrigerating plant 30 according to the invention which is open on both sides is shown diagrammatically.
In Fig. 1b a heat pump 20 which is open on both sides is shown.
The refrigerating plant 30 and the heat pump 20 operate in accordance with the Joule-Brayton process, which in its simplest form consists of an adiabatic compression step, an isobaric heat exchange step and an adiabatic expansion step.
The refrigerating plant 30 which is open on both sides according to Fig, 1a operates as follows:
Air drawn in from the surroundings (atmosphere) is passed to a compressor 32 via inlet 51. The air is compressed in the compressor 32, during which process the pressure and temperature of the air increase. Then, the compressed air is fed via an intermediate conduct system 6, 7, in which a regenerative heat exchanger which will be described with reference to Fig. 3 is incorporated, to an expander 31 (expansion device). Expansion of the air takes place in the expander 31, during which process the pressure and temperature of the air decrease. The expanded, cooled air is fed via conduct 46 to the target room 34 which is to be cooled. This target room 34 which is to be cooled can be an essentially closed room, such as a cold store or a freezer store. The cooled air flows from conduct 46 freely into the cold-storage room 34. During this process, the cold air supplied will spread out in the target room, as a result of which uniform cooling of the target room 34 can be realized. Air from the target room 34 which is still relatively cold or cool is discharged from the target room 34 via return conduct system 4, 5. Before this air which is still relatively cold or cool flows freely out into the surroundings, it is passed through the regenerative heat exchanger for the exchange of heat and moisture with the flow of air which is to be fed to the target room 34. During this process, the air discharged from the target room via the return conduct system 4, 5 takes up heat and moisture from the air which is to be fed to the target room 34.
In Fig. 1b, a so-called heat pump according to the invention is shown diagrammatically. Briefly, the operation of this heat pump is as follows:
Air is drawn in from the surroundings via conduct 25. This air is expanded in an expander 22. During this process, the pressure and temperature of the air decrease. Then the air is fed via intermediate conduct system 6, 7, in which a regenerative heat exchanger which will be described with reference to Fig. 3 is incorporated, to a compression device 23. In the regenerative heat exchanger, the temperature of the air is increased, while the pressure will remain essentially the same. Once it arrives in the compressor 23, the air is compressed, during which process the pressure and temperature increase. The warm air is then fed via conduct 24 to the target room 21 which is to be heated. The heated air flows essentially freely into the target room 21, as a result of which a uniform and efficient heating of the room is made possible. Air is discharged from this target room 21 via a return conduct system 4, 5, which air, before flowing to the outside air via conduct 5, is passed through the regenerative heat exchanger 1 in order to preheat the air which is to be fed to the target room 21 via the intermediate conduct system 6, 7.
Fig. 2a shows another embodiment of a refrigerating plant according to the invention. This refrigerating plant 40 according to Fig. 2a generally corresponds to the refrigerating plant 30 according to Fig. 1a. The difference is substantially that in the refrigerating plant 40 according to Fig. 2a, the compression takes place in two stages. There is a first compression stage, which takes place in the further compression means 41, and a second compression stage, which takes place in the first compression means 33. Preferably, intermediate cooling takes place between the two stages by means of a heat exchanger 43, which is incorporated in the conduct system 42, 44 between the two compressors 41 and 33. In this heat exchanger 43, the air which has been passed through the conduct system 42, 44 is cooled with the aid of ambient air, which is fed to the heat exchanger 43 via conduct 49, and is discharged again from the heat exchanger 43 to the surroundings via conduct 57. The compressor 33 and the expander 31 here form a so-called turbo charger, which is driven by a common shaft 45. Such turbo chargers generally operate optimally at rotational speeds of 50,000 to 150,000 rpm. The extra compressor 41 here makes it possible to supply sufficient work.
Leakage flows can be counteracted by keeping the pressure in the room which is to be cooled essentially identical to the ambient pressure. This can be realized by incorporating compression means in the air flow leaving the target room. Preferably, these compression means are located in conduct 5. The control for these compression means can be based on the pressure difference between the surroundings and the target room or on the volume flow rates in the conduct systems 4, 5 and 51, 42, 44, 6, 7, 46.
The cooling temperature or cooling capacity of a refrigerating plant 40 according to Fig. 2a can advantageously be controlled by controlling the rotational rate of the compressor 41, for example by means of a speed regulator for a rotor.
Fig. 2b shows another embodiment of a heat pump according to the invention. This heat pump 29 according to Fig. 2b largely corresponds to the heat pump 20 according to Fig. 1b. The difference is essentially that in the heat pump 29 according to Fig. 2b the compression takes place in two stages. There is a first compression stage, which takes place in the further compression means 26, and a second compression stage, which takes place in the first compression means 23. Intermediate cooling or heating between the compression stages, such as in the refrigerating plant according to Fig. 2a, is not necessary in the heat pump 29 according to Fig 2b. The compressor 23 and the expander 22 form, in a corresponding manner to the compressor 33 and the expander 31 of the refrigerating plant 40 according to Fig. 2a, a so-called turbo charger, which is driven by a common shaft 28. As has already been explained, such turbo chargers generally operate optimally at rotational speeds of 50,000 to 150,000 rpm. The extra compressor 26 here makes it possible to supply sufficient work.
In the heat pump 29 too, leakage flows can be counteracted by keeping the pressure in the room 21 which is to be heated essentially identical to the ambient pressure. This can be realized by incorporating compression means, such as a fan, in the flow of air leaving the target room 21. Preferably, these compression means are located in conduct 5. The control for these compression means can be based on the pressure difference between the surroundings and the target room 21 or on the volume flow rates in the conduct systems 4, 5 and 25, 6, 7, 27, 24.
The heating temperature or the heating capacity of the heat pump 29 according to Fig. 2b can advantageously be controlled by controlling the air flow rate of the compressor 26, for example by means of a speed regulator for a rotor.
Fig. 3 shows an example of a regenerative heat exchanger 1 according to the invention. This heat exchanger comprises two compartments 2 and 3, which are each provided with a packed bed of particulate material 10. As shown, the particles 11 thereof are essentially spherical, but differently shaped, for example irregular, particles are also very conceivable.
The operation of the regenerative heat exchanger 1 can be described as follows:
Fluid, such as air, which is to be fed to the target room 21, 34, is fed to the regenerative heat exchanger via conduct 6, passed by means of valve 9 to the left-hand compartment 3, passed through the packed bed located herein and fed through in the direction of the target room via valve 8 and conduct 7. Fluid, such as air, which has been discharged from the target room, is passed into the right-hand compartment 2 via conduct 4 and valve 8, passed through the packed bed of particulate material 10, and discharged from the regenerative heat exchanger 1 via valve 9 and conduct 5. By simultaneously rotating the positions of the valves 8 and 9 through 90° to the positions shown in dashed lines, the effect is achieved that the flow of fluid which is going towards the target room is passed through the right-hand compartment (instead of through the left-hand compartment), and that the flow of fluid which is discharged from the target room is passed through the left-hand compartment (instead of through the right-hand compartment). By allowing this rotation of the valves 8, 9 to take place with a regularity to be determined in more detail (which will be dependent on, inter alia, the process conditions, the physical properties of the particulate material, etc.), the effect is achieved that alternately the bed is always taking up heat in one compartment while the bed is giving up heat in the other compartment.
It has been found that the bed of such particulate material is also very able to bind to itself condensation or precipitation, such as deposition of moisture or ice, and then to give it up to the other flow of fluid. In this way, one of the flows of the fluid can be dehumidified. This can bring considerable advantages, in particular when such a regenerative heat exchanger is used in refrigerating plants. The air which is to be fed to the refrigerating plant is hereby simultaneously cooled and dehumidified in the regenerative heat exchanger, it being possible to discharge the moisture through the flow of fluid discharged from the cold-storage room after the valves have been switched over, while the temperature of the particulate material is simultaneously lowered. In this way, the regenerative heat exchanger 1 is prevented from becoming blocked as a consequence of the deposition of moisture and/or ice.
A further advantage is that the flow of air which has been dehumidified in the regenerative heat exchanger 1 will not form any deposition of moisture or ice in further process steps in a refrigerating plant or heat pump either, so that here too the apparatus is prevented from becoming defective.
Fig. 4 shows a regenerative heat exchanger 70 according to the invention of the so-called rotary type. This regenerative heat exchanger 70 consists essentially of a cylinder 74 which is subdivided into compartments extending in the longitudinal direction thereof. The cylinder 74 shown in Fig. 4 comprises three compartments 71, 72 and 73, which are obtained by installing three partitions 75, 76 and 77 in the cylinder 74, which partitions are fixed to one another at the longitudinal centre axis of the cylinder 74. Each compartment thus obtained is filled with a bed 10 of particulate material 11. The cylinder 74 is arranged so as to be rotatable about axis 78. Feed conducts 4, 6 and discharge conducts 5, 7 are arranged at the ends of the cylinder 74. The conducts 4 and 5 here form part of one conduct system, while the conducts 6 and 7 form part of another conduct system. By rotating the cylinder around its axis of rotation 78, while fluid, for example air, is flowing through the conduct systems 4, 5 and 6, 7, respectively, the effect is achieved that the various compartments are successively incorporated in one conduct system (for example 4, 5) or the other conduct system (for example 6, 7). By subdividing cylinder 74 into three or more compartments, it is easy for a compartment to be connected only to one conduct system simultaneously, so that the flows of fluid from the various conduct systems cannot mix with one another directly. The operation of the regenerative heat exchanger 70 is, as will be clear, otherwise in accordance with the operation as has been described with reference to Fig. 3 for regenerative heat exchanger 1. It will also be clear, then, that the regenerative heat exchanger 1 from Fig. 1a, 1b, 2a and 2b can readily be replaced by a regenerative heat exchanger 70 of the rotary type.
It will be clear that many variants are conceivable on the regenerative heat exchanger shown diagrammatically in Fig. 3. Thus, for example, feed conduct 6 can be a discharge conduct, discharge conduct 7 then becoming a feed conduct. Accordingly, it is possible to reverse the direction of flow in the conducts 4 and 5. Instead of valves 8, 9, many other changeover means for making one or the other flow of fluid flow through the respective compartments alternately are also conceivable. A variant is also conceivable wherein the regenerative heat exchanger comprises one, preferably cylindrical, compartment which rotates about its axis. The particulate material in this compartment is then exposed in turn to one or the other flow of fluid.
Numerous variants on the heat pump and refrigerating plant shown in Figures 1a, 1b and Fig. 2 are also conceivable. Thus, heat exchangers, compressors, expanders, etc. can also be connected upstream, intermediately or downstream. In their simplest form, such apparatuses should comprise at least one expansion device, at least one compression device, and at least one regenerative heat exchanger arranged between a compression device and expansion device.

Claims

Apparatus for the transfer of heat with the aid of air, comprising successively in the direction of flow of the air:

first means (22, 32) for changing the pressure of the air in a first sense;

a first part of a regenerative heat exchanger (1) for the exchange of heat from the air in a first direction;

second means (31, 23) for changing the pressure of the air in a second sense, which second sense is opposite to the first sense;

a second part of said regenerative heat exchanger (1) for exchanging heat from the air in a second direction, which is opposite to said first direction;

in which the apparatus further comprises a target room (21, 34) into which heat has to be transferred, which target room is provided with an air inlet and an air outlet,

in which the air inlet of the target room (21, 34) is connected to the second means (31, 23);

in which the air outlet of the target room (21, 34) is connected to the second part of the regenerative heat exchanger;

in which the first means are provided with an inlet (51, 23) withdrawing air exclusively from the surroundings; and

in which the outlet (5) of the second part of the regenerative heat exchanger (1) is connected exclusively to the surroundings,

characterized in

that the first and second parts of the regenerative exchanger (1) are interchangeable with regard to their function and comprise a bed of particulate material for the exchange of sensible heat, latent heat and moisture;

that the equivalent diameter (D) of the particles (11) of the particulate material is between 1 and 25 mm, the equivalent diameter (D) being determined according to D = 3 6V π in which V is the volume of a particle, and that the physical properties of the particulate material (10), the equivalent diameter of the particulate material (10), and the dimensions of the heat exchanger are designed to satisfy the following relationship: p: B = A D λ2m air CpL2 < 0.1 wherein

A =

total heat-exchanging surface area of the particles in one compartment, in m²

D =

average equivalent diameter of the particles in m

λ =

heat transfer coefficient of the particles in W/mK

m_air =

mass flow of air in kg/s

C_p =

specific heat of air in J/KgK

L =

length of bed in m.
Apparatus according to Claim 1, comprising a cooling system, wherein the first means (33) comprises a compressor, the second means (31) comprises an expander, the first part of the regenerator is designed to take up sensible and latent heat from the air and the second part of the regenerator is designed to give up sensible and latent heat to the air.
Apparatus according to Claim 1, comprising a heat pump, wherein the first means comprise an expander (22), the second means comprise a compressor (23), the first part of the regenerator is designed to give up sensible and latent heat to the air and the second part of the regenerator is designed to take up heat from the air.
Apparatus according to one of the preceding claims, wherein the ratio (e) of the volume of the particulate material in the regenerator to the volume occupied by the bed is between 0.2 and 0.8.
Apparatus according to one of the preceding claims, wherein the particles (11) of the particulate material are essentially spherical.
Apparatus according to one of the preceding claims, wherein the particulate material comprises a ceramic material.
Apparatus according to one of the preceding claims 1 to 5 wherein the particulate material comprises glass.
Apparatus according to one of the preceding claims, wherein the particulate material comprises a hygroscopic material.
Apparatus according to one of the preceding claims, comprising further compression means (41; 26), the outlet (42; 27) of which is connected to the inlet (44; 27) of the compression means (33; 23), optionally via a heat exchanger (43) exchanging heat with the surroundings.
Apparatus according to Claim 9, wherein the compression means (33; 23) and the expansion means (31; 22) each comprise a rotor, it being possible for the two rotors to be driven by means of a common shaft (45; 28).
Apparatus according to one of Claims 9 or 10, comprising further compression means, such as a fan, for adjusting the pressure difference between the surroundings and the target room.